This invention relates to biological waste treatment systems and more particularly to an apparatus for treating sewage wastes through consumption of organic matter in the waste materials and reduction of microorganism content prior to effluent discharge.
Sewage wastes, particularly those of human origin, are conventionally treated with aerobic, organic matter consuming microorganisms to obtain a reduction in noxious substances in the sewage. For example, in the well known activated sludge process, raw sewage is commonly fed to one or more treatment chambers where it is agitated in the presence of oxygen and aerobic microorganisms (biological sludge). The microorganisms feed on the organic matter substrate in the sewage, and under a suitable balance of organic matter, oxygen and microorganisms, will substantially reduce the organic matter content of the sewage while growing and significantly increasing in population size. After an appropriate reaction period, the mixed liquor of treated sewage and microorganisms is typically transferred to a settling vessel where the microorganisms are allowed to settle from the mixed liquor to form a concentrated mass of the biological sludge. The supernatant from this settling process is discharged from the system as a "clarified effluent." A first portion of the settled biological sludge is then typically recycled to the treatment chamber to act as a microorganism charge in the treatment of incoming raw sewage, while a second portion of the settled sludge must typically be subjected to further prolonged treatment prior to disposal.
The foregoing activated sludge process, while effective in certain respects, has several disadvantages. For example, a portion of the sludge must be recycled to the sewage treatment chamber, requiring a significant capital investment in recycling equipment as well as continuing maintenance expenditures. In addition, in order to operate efficiently, the ratio of incoming raw sewage to recycled sludge must be carefully controlled, requiring continuous monitoring by skilled personnel of the incoming sewage wastes to control the amount of recycled sludge and maintain the desired operating parameters of the system. In addition, the sludge which is not recycled must be further treated and disposed of in a conventional manner, requiring additional treatment, transportation and disposal expenditures.
In order to overcome the foregoing problems, it has previously been suggested to extend the treatment time of the raw sewage in the presence of oxygen. After the microorganisms have depleted a substantial portion of the organic matter substrate as a food source from the sewage, the microorganisms in the presence of oxygen shift from a phase of growth and reproduction to a phase of autodigestion wherein they undertake endogenous respiration and/or intercell cannibalization, both yielding a reduction in the mass of microorganisms and a reduction in the amount of resulting biological sludge produced by the treatment system. For example, U.S. Pat. No. 3,694,353 of Yang, et al purports to relate to such a system. While the extended aeration process has shown a degree of promise in overcoming the sludge removal and recycling problems inherent in the conventional activated sludge process, prior methods and apparatus for carrying out this process have been found to be lacking in certain respects. For example, the efficiency of obtaining total oxidation of organic matter and biological sludge in a sewage treatment process is highly dependent upon obtaining a thorough and complete, uniform, continuous mixing of mixed liquor and suspended solids in the system with an oxygen containing gas, a problem which has not been solved by prior apparatus designs. In addition, in order to obtain complete aerobic sludge autodigestion, it is necessary to provide means for forcing the microorganism population into the autodigestive phase at a location removed from the organic matter food source, i.e., the input for raw sewage.
It has now been determined that the foregoing problems can be overcome and that a sewage treatment system that provides for complete, intimate, continuous contacting and intermixing of oxygen throughout mixed liquor in the system and completely eliminates the need for biological sludge recycling and routine biological sludge removal is obtained by methods and apparatus comprising a series of treatment units, each unit comprising an elongated sidewall of generally circular cross sectional configuration, first and second endwalls mounted in fluid tight engagement with opposite ends of the sidewall to form a generally cylindrically shaped sewage treatment chamber therewithin, first and second baffle means vertically mounted in spaced relationship in the sewage treatment chamber longitudinally of and on opposite sides of the longitudinal central axis of the sidewall and in spaced relationship relative to the sidewall for directing the flow of mixed liquor within the sewage treatment chamber; gas supply means for supplying an oxygen containing gas to the treatment chamber at a location intermediate the first and second baffle means in a lower portion of the treatment chamber to cause mixed liquor in the treatment chamber to rapidly rise vertically upward between the first and second baffle means and then circumferentially outward and downward in a semicircular flow path defined by the interior surface of the sidewall to the bottom of the treatment chamber and then vertically upward again between the first and second baffle means; inlet means for supplying raw sewage to the first unit of the series; outlet means for withdrawing clarified effluent from the last unit of the series; and fluid communication means for maintaining an upper liquid level in the treatment chamber of each unit of the series and for providing serial fluid communication between an intermediate top portion of each successive unit of the series.
The system provides for a uniform, complete and thorough intermixing of mixed liquor and suspended solids throughout each treatment chamber thereby eliminating the settling of any biological sludge in the apparatus, and additionally provides for uniform dispersal and intimate contacting of oxygen in the gas with mixed liquor and suspended solids throughout the treatment chamber thereby providing a suitable environment for continuous aerobic activity of microorganisms in the mixed liquor. Serial fluid communication between units of the series provides an environment suitable for organic matter substrate consumption and microorganism generation and development in an initial unit of the series adjacent the raw sewage inlet means, an environment suitable for microorganism population maintenance in an intermediate unit of the series, and an environment suitable for forced autodigestion of the microorganisms in latter units of the series, thereby eliminating biological sludge produced in the microorganism development phase.